#include <bits/stdc++.h>

using namespace std;

ifstream fin("sate.in");
ofstream fout("sate.out");

int N, M, Nst, Nend;
int distanta[30005];
vector<pair<int, int>> G[30005];

int BFS(int NodStart) {
    queue<int> q;
    distanta[NodStart] = 1;
    q.push(NodStart);

    while(!q.empty()) {
        int Nod = q.front();
        q.pop();

        if(Nod == Nend) {
            return distanta[Nod] - 1;
        }

        for(auto vecin : G[Nod]) {
            if(!distanta[vecin.first]) {
                distanta[vecin.first] = distanta[Nod] + (Nod > vecin.first ? -vecin.second : vecin.second);
                q.push(vecin.first);
            }
        }
    }

    return -1;
}

int main()
{
    fin >> N >> M >> Nst >> Nend;
    for(int i = 0; i < M; ++i) {
        int x, y, cost;
        fin >> x >> y >> cost;
        G[x].push_back({y, cost});
        G[y].push_back({x, cost});
    }

    fout << BFS(Nst);

    return 0;
}